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| A type of instability seen in feedback ac-coupling of in-amp |
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| alishirali:
Dear all, I'm just testing the idea of ac-coupling an instrumentation amplifier (hear I tested both AD620 & AD8220) by a feedback from output to reference pin. The circuit and output are attached. In the test experiment, non-inverting input (IN+) is connected to VGND and inverting input (IN-) is derived by a small amplitude pulse around VGND (blue line). The pulse is generated by a 555 IC then buffered and damped by an LM358 IC. V_CM is O.C. Output and input are plotted red and blue respectively, with the aim of Arduino serial plotter. VCC is connected to 5V battery. The problem: as it was expected, the rectangular pulse input is amplified and high passed exactly with the 1/(RF*CF) HPF (I imported data for different frequencies to Matlab and checked the spectrum). The problem is the output envelope which looks like a first order circuit charge and decharge and I can not stop it from oscillation. I have simulated the circuit in PSpice and no such effect exists there. The problem get better but not solved if I replace AD8220 by AD620. I have tested different ideas to solve this problem but non of them was successful. Any idea on the cause of the problem and its possible solution will be greatly helpful for me! Thanks is advance :D |
| duak:
You have accidently designed an oscillator. U2A forms an integrator that has a 90 degree phase shift. U1 will also introduce a phase shift and at some frequency, the two will combine to give net positive feedback. You might be able to cure this by adding a resistor in series with CF to alter the phase shift at mid to high frequencies. Alternately, you might try to reduce the gain of U1 and then add another gain stage outside of the feedback loop. |
| dmills:
Not sure it isn't U2B in combination with the cap load formed by C_T1, C_T2 causing the honk, opamps really do not in general get on with capacitive loads. Regards, Dan. |
| alishirali:
Tnx Duak, I can not persuade myself that U1 and U2A are forming an oscillator but I tested your idea and the results are as follows. First I reduced the gain by increasing RG from 1k to 10k, keeping all other values unchanged. Then for the case of lowest gain (RG = 10k) I added a resistor in series and parallel to CF. As the results show, I think non of these helped me to avoid the oscillation. |
| alishirali:
Tnx Dmills, I removed C_T1 and C_T2 and posted the results in reply to Duak. No significant change occurred and the output still oscillates. |
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